1. Field of the Invention
The present invention relates to a brake booster apparatus and more particularly to a brake power assist apparatus of a vacuum type.
2. Prior Art
A brake booster apparatus of a vacuum type as shown, for instance, in FIG. 4 is used in general for a brake booster apparatus for a vehicle. Namely, as shown in FIG. 4, in a booster (a) of a vacuum type provided between a brake pedal (p) and a master cylinder (m), the interior of a shell thereof is divided into a constant pressure chamber (b) and a variable pressure chamber (c) by a diaphragm (d), and when the brake pedal (p) is not operated, a vacuum from a vacuum source such as an intake manifold is led into both the constant pressure chamber (b) and the variable pressure chamber (c), and thus the diaphragm (d) remains unoperative. On the contrary, when the brake pedal (p) is depressed, a valve mechanism, not shown, is opened via an operating rod (e), an outside air (atmospheric pressure) is introduced into the variable pressure chamber (c) to thereby produce a pressure difference between the variable pressure chamber (c) and the constant pressure chamber (b), and the diaphragm (d) is sucked and actuated forward by this pressure difference. Then, the operating force of the diaphragm (d) is added to the depression force of the brake pedal (p), and a push rod (not shown) then forcibly actuates a piston (not shown) of the master cylinder (m).
As described above, since the booster of a vacuum type is constructed such that the pressure difference between the constant pressure chamber (b) and the variable pressure chamber (c), i.e., the force sucking the diaphragm (d) is added to the pedal (p) depression force to thereby produce a force for pushing the piston of the master cylinder (m), even female and aged drivers, who tend to be regarded as not being strong, can operate the brakes relatively easily.
A booster of a vacuum type as described above was developed and is disclosed in Japanese Unexamined Patent publication No. Toku-Kai-Sho 61-16305, and in this disclosed booster, a variable throttle valve is provided at a position along an inlet passage of the atmospheric pressure into a variable pressure chamber (c), and the throttling of this variable throttle valve is regulated via a controller by operating a manual switch to close it.
The way to depress the brake pedal and depression force applied thereto depend on drivers, and a preferable braking performance relative to a depression force applied to the brake pedal also depends on drivers. Thus, the prior art described above is effective in that the performance of the booster is suitably set depending on the habit or preference of drivers themselves in which the degree of throttling the variable throttle valve is regulated by operating the manual switch.
However, with a view to optimizing the braking feeling, for instance, if the area of the atmospheric pressure passage is made small and the inlet volume per unit time of the atmospheric pressure is set so as to be small over the whole brake operating areas (i.e., the response of the booster is reduced), although a good brake feeling can be attained in the normal brake operating area, there is caused a delay in obtaining a desired braking performance in panic braking, this causing in turn a risk of lengthening a distance required to bring the vehicle to a halt. On the contrary, with a view to mainly improving the response in panic braking, if the inlet volume per unit time of the atmospheric pressure is set slightly large, there is caused a slightly small problem with the brake feeling in the normal using area.